This invention pertains generally to continuous wave (CW) radar systems having a capability to determine range, and particularly to circuitry for accurately controlling the frequency modulated ranging waveform required by such systems.
As is known in the art, one capability of a CW radar system is the ability to measure, without ambiguity, the velocity of moving targets even though clutter may also be present. A CW radar is, however, incapable of measuring range unless some provision is made to modulate the transmitted signal. One known modulation technique for providing a CW radar with a ranging capability is to frequency modulate (FM) the transmitted CW carrier signal. In known systems utilizing such a technique, the deviation of the FM transmitted signal is compared to that of an FM standard whose deviation is precisely known. Any resulting error then is used to generate a correction signal, changing the deviation of the FM transmitted signal to null the resulting error. The accuracy of any such implementation is, of course, directly related to the accuracy of the FM standard. Unfortunately, however, it is difficult to maintain the requisite accuracy of known FM standards, especially when such a standard must be used in adverse conditions such as are experienced in field use.
One known CW radar system with ranging capability is shown in U.S. Pat. No. 4,176,351 to DeVita et al. According to the cited patent the transmitted signal is frequency modulated at a low modulation frequency and peak deviation. Each received signal then is correlated with each different one of a plurality of differently delayed replicas of the transmitted signal to form a set of signals defining a cross-ambiguity function with a shape corresponding to the J.sub.O Bessel function. Finally, the individual signals in the set of signals are processed in a digital computer in accordance with Doppler and range algorithms to derive estimates of Doppler velocity and range of individual targets.